1. Field of the Invention
This invention relates generally to electrically conductive materials and more particularly to such materials containing silver and metal oxides.
2. Description of Related Art
Silver based composites containing an insoluble second phase material in a silver matrix are conventionally used for electrical contacts due to their performance characteristics relating to contact welding and erosion rates. Silver-cadmium oxide, silver-graphite, silver-tungsten, silver-nickel and silver-tin oxide are examples of such materials as well as certain copper based composites such as copper tungsten. With the exception of silver-cadmium oxide and silver-tin oxide which can also be made by a method involving internal oxidation, composite materials are made by powder metallurgical techniques. The electrical performance of such contact materials depends on the quality and chemical composition of the second phase material, its grain size distribution and homogeneity in the silver matrix. These factors also determine the density of the material after the sintering operation. When the material is used with modern high production rate presses the powder must also be free flowing.
Properties of the insoluble second phase material and its interaction with the silver matrix provide a unique electrical contact performance. Every time an electrical contact is engaged with a mating contact to close an electrical circuit or disengaged to open such a circuit an electric arc is ignited. The intensity of the energy of these arcs melt the silver matrix at the arc spots on both respective contacts.
The metal oxides forming the insoluble second phase material interact with these arcs in three major ways. First, the arc thermally decomposes the metal oxides. By way of example, in the case of cadmium oxide and tin oxide they are decomposed according to the following formulas: EQU CdO.rarw..fwdarw.Cd+O and EQU SnO.sub.2 .rarw..fwdarw.SnO+O
The energy required for this decomposition is on the order of over a thousand joules per gram of oxide. The withdrawal of this energy cools off or "quenches" the arc. A cooler or quenched arc vaporizes and expels less molten silver and therefore diminishes erosion of the contact material.
Second, the thermal decomposition reaction of the oxides influences the strength of welds that always occur when the contacts close. An electric arc which ignites prior to the closing of contacts forms a molten silver pool. Molten silver, which dissolves approximately one hundred times more oxygen than solid silver, rapidly dissolves the oxygen that the thermal decomposition reaction generates. Following the closure of the contacts the solidifying silver pool expels the excess oxygen. As a result, the weld nugget becomes very porous and mechanically weak and breaks easily when the circuit is switched off and the contacts opened.
The third interaction relates to the effect of the metal oxides on the so-called "spitting" type erosion. The molten silver pool at the arc spots swirl rapidly under the forces of alternating magnetic fields. The magnetic force agitates the surface and forms rapidly moving standing waves and expels molten silver droplets from the molten surface. Loss of this silver is a major factor in contact erosion.
Metal oxide particles increase the viscosity of the melt and reduce the swirling and wave motions. As a consequence the molten pool loses fewer silver droplets and the spitting erosion is reduced.
However, molten silver must wet the metal oxide for all three mechanisms to be operative. Without wetting the metal oxide particles are ejected from the silver pool and deposited on the silver surface. Under such a condition arc quenching may still be operative, at least initially, however, the other two mechanisms, namely oxygen absorption by the melt and the melt viscosity are adversely effected. Decomposition of the oxide now occurs on the surface and the molten silver pool cannot readily absorb the evolving oxygen. Weld forces increase and the likelihood of failure due to strong welds increases. The melt contains fewer metal oxide particles and therefore the viscosity decreases and spitting type erosion increases. In addition to the above, the oxide layers that accumulate on the surface increase the contact resistance and lead to over-heating of the switching device.
Although molten silver readily wets some oxides it does not wet certain other oxides such as pure tin oxide. It is known to coat tin oxide powder, as shown in FIG. 1, with a dopant such as copper oxide, bismuth oxide, tellurium oxide, tungsten oxide, and molybdenum oxide to lower the surface tension and promote wetting by molten silver. Silver powder is added to the oxide powder and mixed in a suitable mixing machine. It is conventional to make strip or wire shapes by extruding and rolling billets made with these mixtures. Such processing, however, tends to break the tin oxide particles and in so doing exposes undoped tin oxide surfaces as noted in FIGS. 2a, 2b and 2c. The dopant coating is also striped off particles by the electric arc generated during contact closing and opening to further expose pure tin oxide surfaces. Molten silver wets such particles poorly and tends to expel them from the molten pool as previously described. Materials produced with such doping methods end up increasing welding and erosion tendencies.